MOLLUSCS

Phylum Mollusca

The molluscs form the largest and most diverse group of marine animals on the planet and probably the most familiar group of invertebrates. The family includes nudibranchs or sea slugs, chitons, snails, bivalves, octopus and squid. There are some 75,000 species in eight different families found in the world’s oceans. Most of the species have common characteristics such as an external shell, a soft body with a muscular foot and a rasping, tongue-like radula for feeding, with gills of some kind for respiration. All lay eggs and of the eight groups, three are fairly common around these shores.

CEPHALOPODS

The cephalopods are by far the most successful of the Mollusca and octopus are common throughout the Mediterranean. Extremely intelligent and adaptable, they are free-swimming, fast-moving predators with well-developed eyes and sensory systems. All of the cephalopods are able to change their shape and coloration to suit their environment or to exhibit and display to a mate or a would-be predator. They are also considered prized catches and feature in most coastal seafood restaurants!

MUSKY OCTOPUS

Eledone moschata

Usually mistaken for the Common Octopus, this species is fairly common in shallow waters around the western Mediterranean. With a body length of 15cm (6in) and arms to around 40cm (1ft 4in), it is predominantly brown in colour with darker brown spots arranged irregularly over the body and webs. The animal is distinguished by the single row of suckers. The common name comes from the musky smell it exudes when taken out of the water. The female of the species lays around five hundred eggs about 10mm (½in) long.

WHITE-SPOTTED OCTOPUS

Octopus macropus

The White-spotted Octopus is more commonly seen at night when it is an active hunter. It appears to be exclusively nocturnal and there are virtually no records of the species being seen during the day. The body grows to around 15cm (6in) and the arms are over 1m (3ft 3in) long. It is recognized by the pairs of white spots down the long arms and an overall reddish-brown body.

COMMON OCTOPUS

Octopus vulgaris

Common indeed, this species of octopus is very wide ranging and is found and commercially fished all round the Mediterranean. Over 20,000 tonnes are taken annually off the North African coast alone. With a total body length of around 1m (3ft 3in) and maximum arm spread of 3m (10ft) in diameter, the body is stout and warty in appearance with two rows of suckers found on the arms. The animal preys on crabs and lobster, and will also eat shellfish. The den of the octopus is usually discovered by the amount of shell debris found outside its lair.

COMMON CUTTLEFISH

Sepia officinalis

This species grows to over 45cm (1½ft) long and has an internal cuttlebone, which is a chambered, gas-filled shell used for buoyancy control. Possessing eight arms and two longer feeding tentacles, adults of the species are recognized by the white bands across the bodies and small white spots on the upper surface of the fins. The arms have four rows of suckers and it actively stalks its prey around Posidonia beds and inshore reefs. Like all species, it is characterized by its ability to change colour and body texture at will.

CALAMARI SQUID

Loligo vulgaris

Growing to over 75cm (2½ft) long, this is the most common squid found in the Mediterranean. The species inhabits the water column to all depths, but comes into shallow coastal waters to spawn. The female lays around 20,000 eggs in pale white or grey gelatinous strings attached to the sea floor or hard objects. This squid is caught commercially by a variety of trawl nets and with night lights and jigs.

GASTROPODS

Gastropods form the largest class of the Mollusca family and the body form is widely varied. It includes all univalved shellfish such as limpets, periwinkles and snails. They usually have a shell of one piece and live mostly on the shore or sea bottom. This group also includes the opisthobranchs, which have no shell at all.

CHITON

Chiton olivaceus

This ancient species of animal is particularly distinctive by its eight hinged overlapping plates, which allow the animal to fold itself up. Oval shaped and tinged with various shades of green, it is able to clamp onto rocks with the aid of a strong muscular foot. Common on the shoreline and surf zone, it enjoys aerated water and grows to a maximum size of around 5cm (2in).

QUEEN SCALLOP

Aequipecten opercularis

This small scallop is highly prized in the fish market and is commercially harvested throughout the Mediterranean and Atlantic. It grows to around 9cm (3½in) in diameter and has twenty, sculpted, bold ridges on each side. It lives on mud and soft, sandy seabeds. Ecology: The species usually gets overgrown by a variety of sponges and it is extremely mobile when danger threatens. With the aid of its muscular joint, it opens and closes the two halves of the shell rapidly, thus providing a jet-propelled escape.

WHELK

Buccinulum corneum

This gastropod mollusc has a thick, spindle-shaped shell with an oval opening and an upturned tip where its eye stalks protrude. Usually pink and beige in colour, it grows to around 6cm (2½in) and enjoys grazing amongst Posidonia and algae-encrusted rock surfaces.

MEDITERRANEAN CERITH

Cerithium vulgatum

This tower shell grows to 6.5cm (2½in) and has a thick and broad sculpture with prominent ridges at the growth lines. It has quite a large aperture and thick, flared lips. It is common in soft sand and shell debris areas where it filter feeds. Ecology: This species is a common home for Hermit Crabs and you can often see large numbers of them together on a dead fish or crab.

TRITON TRUMPET

Charonia tritonis

The largest of the trumpet shells and quite rare in the Mediterranean, it feeds on starfish and is an active predator. It can grow over 40cm (1ft 4in) and is constructed of irregular spires, often overgrown by calcareous algae. The foot is beige-spotted with brown, and the eye stalks have two dark bands.

MEDITERRANEAN CONE SHELL

Conus mediterraneus

Distinctively shaped, this cone shell can grow to 7cm (2¾in). The shell is typically broadest at the abapical end of the slit-like aperture, which tapers towards the open siphon canal. It has a short spire with only three or four whorls and the entire shell is smooth and shiny with mottled brown striations.

MEDITERRANEAN COWRIE

Cyprea lurida

Fairly common on night dives, this species of cowrie grows to around 3cm (1¼in) and is found in central and eastern areas of the Mediterranean. It has brownish, broad bands across its back and at both ends. The mantle of the animal is also brown with spiky protuberances and when fully extended is difficult to spot.

SPOTTED COWRIE

Cyprea spurca

This colourful cowrie is relatively rare as it is a night-time predator, preferring to hide well away in a rocky crevice during the day. More likely to be found in a cave situation, this cowrie is tinged orange and rusty brown and eats various types of sponge and bryozoan. The shell is kept smooth and shiny by the animal’s mantle skirt, which completely covers the shell whilst moving.

ABALONE

Haliotis lamellosa

Confined to the Mediterranean, the abalone shell grows to 7cm (2¾in) and enjoys a shaded position, more often than not hiding under rocks during the day. It is recognized by its low spire and limpet shape, and the outer surface has a keel of small round holes. The fleshy foot is not only able to attach itself firmly to a rock, it also allows for easy propulsion. The species feeds on red algae. The inside of the shell is of iridescent mother of pearl.

TUBE SNAIL

Lementina arenoria

This is a curious mollusc of a diverse family with nine species in total to be found, which are easily confused with each other. Resembling a tube worm, the Tube Snail has a wide obvious shell and can grow up to approximately 1.5cm (½in) in diameter, often overgrown with algae and small corals. The animal has no operculum and the mouth of the tube shows the reddish-brown snail streaked with a golden-yellow pattern. It lives inside this tube all the time and fishes for prey by extending a mucous net, capturing plankton and transporting it to the gut by ciliary currents produced by its gills.

SPINY FILE SHELL

Lima lima

This file clam always lives in a rocky crevice and is usually seen when it extends its white, sticky feeding tentacles from the mouth of the shell. With a shell of equal halves, it soon grows large enough to be unable to move from its home and will eventually die in the some location. Those found in the open are able to swim in the same way as a scallop does, by expanding and contracting a strong muscle, opening and closing the shell rapidly, providing propulsion.

COMMON MUSSEL

Mytilus edulis

Located extensively in shallow water, particularly around the surf zone, mussels are a highly prized addition to the Mediterranean seafood diet. Commercially farmed in a number of areas, wild mussels are coloured a dark bluish-purple and are able to attach themselves together in large groups by means of a renewable byssus or stiff secreted thread. They grow up to 5cm (2in) long and there are some 36 species found in European seas.

GOLDEN CARPET SHELL

Paphia aurea

This small bivalve is found in large numbers in rough sand and shell gravel and is harvested mercilessly. Growing to 3cm (1¼in) across and roughly triangular in shape, the concentric sculpture has obvious growth rings and is overlaid with an intricate zig-zag pattern.

NOBLE PEN SHELL

Pinna nobilis

Resembling a partly folded fan, these giant mussels are highly sought after as souvenirs and are a protected species in the Mediterranean. In special circumstances, the shell can attain a height of over 1m (3¼ft). It prefers to live amongst Posidonia. The outer shell is quite rough and makes an ideal home for many different species of algae and sponge and it also harbours a few commensal crabs and shrimps within its body cavity. In the 19th century, the byssus threads were collected, dried and then woven into gloves, shawls and stockings, similar in texture to silk. These products were greatly sought after by the upper classes.

ROUGH PEN SHELL

Pinna rudis

Much smaller than its cousin, this pen shell grows to around 10cm (4in) across and has a much flatter anterior lip. This fan mussel prefers very low light conditions and a hard rocky substrate where it grows from a crevice. Also found on rough gravel sea floors at the entrance to caverns, this rare species has delicate raised whorls along the outside of the almost translucent shell.

SMOOTH CARPET SHELL

Pitar rudis

Extensively collected for public consumption, the Smooth Carpet Shell, up to 2½cm (1in) long, has a shiny outer surface coloured in shades of beige and golden brown with darker concentric circles of colour. It prefers a soft sand or muddy seabed, where this species can occur in vast numbers.

WINGED OYSTER

Pteria hirundo

This very distinctive oyster has equal valves to the shell and is usually found attached to gorgonian sea fans, where it enjoys the strong current to filter-feed passing plankton. Utilizing byssus threads to attach itself to the sea fan, this shell grows to 6cm (2½in) in diameter, vaguely oval in overall shape but with a distinct thin ear or wing.

ROCK SCALLOP

Spondylus spp.

This species is probably Spondylus gaederopus, but as always with these bivalves it is difficult to identify owing to the covering of distinctive raised spines on the shell covered by the sponge Crambe crambe. The bottom half of the shell is firmly adhered to the rocky surface and reaches 12cm (5in) in length. Preferring caves and low light conditions, this scallop has its distinctive double row of eyes around the body cavity and pale-cream inner muscle. Highly prized both for its meat and pearl producing capabilities, the species is in decline throughout the Mediterranean.

TRUNCATED WHELK

Trunculariopis trunculus

Growing to around 8.5cm (3½in), this whelk has a very rough exterior shell. It grazes on various algae and, although it is often seen singularly, it can form large breeding groups and is known to mass spawn. With only a few concentric whorls of the shell, the anterior is markedly upturned and protects the eye stalks.

WARTY VENUS

Venus verrucosa

This is a stout bivalve with a shell of equal halves formed of more than twenty concentric growth rings. Growing to around 6cm (2½in) in diameter, it is yellowish-white or orange in colour and lives partly buried in rough sand and shell gravel. This species is highly prized by octopus, which often use the shells to seal the entrance to their den.

SEA HARES

Herbivorous sea hares are characterized by rather fleshy bodies, which are swollen towards the rear and a head, which is split into pairs of lobed oral and sensory tentacles. Sea hares have the ability to release clouds of purplish ink as well as milky white, sticky threads as a means of defence.

GIANT SEA HARE

Aplysia fasciata

Growing to over 40cm (1ft 4in) and with a weight of nearly 2kg (4½lb), the Giant Sea Hare is a dark peaty brown in colour with a mauve tinged edge to the skirt. It feeds on a large variety of seaweeds and is usually found in pairs.

SMALL SEA HARE

Aplysia punctata

Growing to only 20cm (8in), half the size of its cousin, the Small Sea Hare is also a much lighter brown in colour, although this colour varies depending on the diet. It is delicately marked with lighter spots over a mottled or blotched body.

PLEUROBRANCHS

Pleurobranchs still have the vestiges of a shell, which may be either internal or external. They have acid-secreting glands for defence and most species are carnivorous.

UMBRELLA SNAIL

Umbraculum mediterraneum

This is a very distinctive oval slug, 4cm (1½in) long. It looks as though it is carrying an algae-covered limpet as a hat and owing to the hundreds of lobed portions to the body, is sometimes mistaken for a shell with its egg cases. It has two short rhinophores, or sensory tentacles, at the head and enjoys light and well-aerated water.

NUDIBRANCHS

The name ‘nudibranch’ quite literally means ‘naked gills’ and these are snails, which have lost their shells. This is the largest order of Opisthobranchs and the body of the various species varies widely, but they also have similar characteristics in that they usually have obvious rhinophores at the head and a set of gills at the rear of the body. They may have various projections dotted about and all are carnivorous.

WHITE-TIPPED NUDIBRANCH

Coryphella pedata

This species grows up to 5cm (2in) long and has seven paired clusters of cerata, all of which are white tipped. The body is a uniform purple and may have some light spots along its back. It feeds on various hydroids including Eudendrium. Not to be confused with Purple Nudibranch.

MIGRATING AEOLID

Cratena peregrina

This species is quite common in central and northern regions, where it can occur in quite large numbers, depending on the time of year. It has six clusters of cerata merging to brown with a purple tip. The oral tentacles are tinged in blue to white and there is a distinct orange flash on the head between the rhinophores. It grows up to 5cm (2in) long.

SEA LEMON

Dendrodoris limbata

Typically oval in shape and lemon to brown in colour, it has a blotched body with a yellow frill. This species is quite small, only growing to 2.5cm (1in) and inhabits deeper water where it feeds on a variety of hydroids, bryozoans and even sponges.

WHITE LINED DORID

Doriopsilla pelseneri

Another distinct species, this dorid is a golden yellow in colour and grows to 3cm (1¼in) long. Oval in shape, the body is covered in a tracery of fine white lines. Both the rhinophores and gills are the same colour as the body and are retractile.

ELYSIA

Elysia viridis

This colourful and delicate opisthobranch grows to around 2.5cm (1in) in length and has rudimentary eyes situated behind the rhinophores. It tends to be shades of green in colour owing to the absorption of the chloroplasts from the algae that it eats. This common species has been observed laying eggs and eating the invasive Caulerpa racemosa.

PURPLE NUDIBRANCH

Flabellina affinis

The Purple Nudibranch is different from Coryphella pedata (see here) in that it does not have the white tips to its cerata. A garish violet colour, it is also deeply tinged with red and lives on the polyps of hydrozoa. Growing up to 5cm (2in) long, it has six to nine paired clusters of cerata and its rhinophores are also the same as the body colour. It feeds on species of the hydroid Eudendrium.

LADY GODIVA

Godiva banyulensis

Quite common throughout the region, this aeolid has a beige body with distinct white lines down each side of its back and outside of the skirt. The oral tentacles are tinged with blue and the seven clusters of cerata along the back are tinged with fawn and brown with a golden tip. It grows up to 5cm (2in) long.

ELEGANT SEA SLUG

Hypselodoris elegans

This is one of the largest nudibranchs to be found in the Mediterranean, growing to over 15cm (6in), and it comes in a variety of body colours, all of which are rather bright and garish. Ranging from brilliant yellow to violet and even green, the gill tuft is very prominent, almost like a flower. The body mass has a frilly edge and the species enjoys fairly deep water, feeding on encrusting hydroids and coral polyps. There would appear to be some confusion over this species of Hypselodoris as it is known by various other names, depending on the colour pattern. Scientists are still not convinced that these other colour forms are separate subspecies, rather than just colour changes due to the different diets from various areas of the Mediterranean. Names that you will come across in various regional guide books are H. picta, H. webbi and H. valenciennesi, but it is generally assumed that they are all the same species.

TRI-COLOUR DORIS

Hypselodoris tricolor

Much smaller in size, this species grows to only 2cm (¾in) and is a brilliant blue in colour with equal horizontal yellow lines and another yellow line completely encircling the body. The rhinophores are dark blue, as are the gills.

CRYSTAL TIPS

Janolus cristatus

This species has an oval, flattened shape and is fringed all round the 7.5cm (3in) body by golden cerata or tentacles, each tipped with white and iridescent blue. The rhinophores are white and non-retractile. This species feeds on bryozoans. Widely distributed throughout the Mediterranean, Canary Islands, Bay of Biscay and the British Isles, it is an active night-time predator. Crystal Tips can be found on many different substrates, preferring clean, shallow and sheltered waters. It is more often than not seen wandering over the sandy seabed in search of another meal.

SPOTTED DORIS

Discodoris atromaculata

Particularly distinctive in appearance, with a light-coloured body and dark brown, irregular spots; usually growing to around 12cm (4¾in) in length, it is often quite smaller. The eight gills and rhinophores are both white. This species feeds almost exclusively on the sponge Petrosia ficiformis and can be found in open, well-lit areas as well as deep caverns and caves.

LEATHERY DORIS

Platydoris argo

Burnt orange or brown in colour and roughly oval in shape, the outer skirt of the body is deeply ruffled, giving it a rather indistinct outline. This curious doris is always found in association with a partner or partners, usually moving along in pairs, just slightly overlapping. It can grow to 10cm (4in) long and its rhinophores and gills are deeply set back from its frame and are totally retractile.

FOUR-LINED NUDIBRANCH

Polycera quadrilineata

A common sea slug in the western Mediterranean, this species is an Atlantic immigrant and is common throughout the Bay of Biscay and up to Scandinavia. It has a distinct white body with yellow stripes around the frame but may have lots of black spots and lines and has four prominent yellow-tipped oral tentacles. It grows to 3.5cm (1½in) and feeds on a variety of bryozoans and hydroids.

CEUTA SEA SLUG

Tambja ceuta

Largely confined to the Straits of Gibraltar and the Moroccan coast, this species is a dull greyish-green or blue in colour with darker blue or violet lines and pale yellow along the body. It grows to over 4cm (1½in) long.